Quite a number of you requested tutorial for this, so here is a photo to refresh your memory
Keep in mind, a lot of this could be simplified by using a T-shirt as a base as opposed to fabric...this would reduce the process to two ridiculously easy steps (adding a new neckline/hood and elongating the body!)
Also, this tutorial does not include how to keep all the seams concealed to the interior, so you're on your own as far as that goes; hint: use double layers of fabric (printed sides facing out) on the top part and hood so that the stitching is hidden. (remember, different parts of it will be showing depending on what your hood is doing, so in other news does anyone know what language I am speaking?)
Ok I'm pretty sure I am complicating your life with all this post scriptum so I'll just leave it at that, thank you for looking and enjoy! or contort your face in complete confusion, which ever!!